Time-Resolved Powder X-ray Diffraction of the Solvothermal Crystallization of Cobalt Gallate Spinel Photocatalyst Reveals Transient Layered Double Hydroxides

Abstract

The use of in situ X-ray diffraction to study the solvothermal crystallization of a gallium oxide spinel material was studied. Two experiments are presented, one the spinel formation in a water-monoethanolamine (MEA) mixture and the formation in only MEA. The photocatalytic properties of the spinels synthesized from both reaction conditions are then presented. In the reaction of gallium metal and cobalt(II) nitrate hexahydrate (2:1 ratio) in monoethanolamine (MEA) and water mixture (1:1 by volume) at 210 °C, transient phases with complex growth and decay profiles precede the formation of the expected cobalt gallium oxide. The first period of growth and decay of the transients can be seen to occur within the first 45 min of reaction with a decreased d-spacing of its Bragg reflections. A similar reaction conducted using solely MEA as the solvent also formed a cobalt gallium oxide spinel with onset of crystallization after around 150?175 min. However, in this case there was no formation of any crystalline intermediate. Off-line experiments with ODISC were used to attempt to isolate the transient phase(s). The powder X-ray diffraction (PXRD) pattern of the isolated solid after approximately 80 min is in good agreement with the in situ data after the same reaction time. The fast acquisition time of the in situ XRD uniquely allowed for observation of two transient phases including one in particular which is very short-lived and unlikely to have been quenched and seen by conventional laboratory techniques.